Cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) has a structure which is commonly viewed as a two-dimensional framework. Thus, in cordierite, two-dimensional, co-planar sheets consisting of hexagonal rings of tetrahedrally-coordinated cations are perpendicular to the c-axis. One aluminum atom is present in each ring. Alternate layers of the hexagonal ring structure are connected such that two cavities per unit cell are defined by three co-planar Si.sub.5 Al rings and the network of Mg octahedra and Al tetrahedra which link the rings. The overall configuration of the framework resembles spheres sliced and ground to yield two hexagon flats per sphere and a repeat unit of two spheres, one above the other, per unit cell. A fundamental feature of this structure is that it contains cavities which are assumed to be unoccupied in the pure material. Cordierite has a positive axial thermal expansion coefficient in the plane of the hexagonal rings and a negative axial thermal expansion coefficient perpendicular to that plane.
Numerous chemical derivatives of cordierite have been synthesized. Those products have generally involved additions of Al.sup.+3 ions for Si.sup.+4 ions, thereby changing the Al.sup.+3 /Si.sup.+4 ratio and resulting in ring charge imbalance, this charge imbalance being compensated for through an interstitial cation located in a structural cavity. Thus, additional Al.sup.+3 ions have been substituted on a statistical basis for Si.sup.+4 ions in the rings and positive charges have been introduced via the addition of monovalent or divalent metal ions to maintain charge balance. Three illustrations of that practice are set out below.
U.S. Pat. No. 3,926,648 discloses the addition of 0.5-2.5% by weight K.sub.2 O and/or Cs.sub.2 O to glass compositions near the stoichiometry of cordierite to improve the sinterability of such compositions when fine powders thereof are fired to glass-ceramic bodies. Because the inventive products were sought to demonstrate physical properties similar to those exhibited by cordierite, e.g., a coefficient of thermal expansion (25.degree.-1000.degree. C.) of 13-18.times.10.sup.-7 /.degree.C., a use temperature of 1300.degree. C., and good thermal stability when cycled between room temperature and 1000.degree. C., the maximum total of K.sub.2 O and/or Cs.sub.2 O was emphasized to be 2.5%. Stated in other words, the addition of small amounts of K.sub.2 O and/or Cs.sub.2 O permitted the sintering together at lower temperatures of glass powders having the approximate stoichiometry of cordierite into strong glass-ceramic bodies wherein the crystal phase retains the essential physical properties of cordierite.
U.S. Pat. No. 4,015,048 describes the addition of 0.7-5.6 mole percent of a divalent metal oxide selected from the group of BaO, CaO, PbO, and SrO to glass compositions near the stoichiometry of cordierite to improve the sinterability of such compositions when fired as fine powders into a glass-ceramic article. The fired article is non-porous, essentially free from glass, and exhibits a coefficient of thermal expansion (25.degree.-1000.degree. C.) of not over 20 .times.10.sup.-7 /.degree.C.
U.S. Pat. No. 4,194,917 teaches the production of a fired dense ceramic body consisting essentially of 0.3-8% by weight Y.sub.2 O.sub.3 and/or CeO.sub.2 in cordierite. The additions improve the density of the fired product and impart a low coefficient of thermal expansion thereto, i.e., &lt;30.times.10.sup.-7 /.degree.C. (25.degree.-1000.degree. C.).
When divalent metal ions are utilized to maintain charge balance in those instances where Al.sup.+3 ions are substituted for Si.sup.+4 ions in the cordierite structure, their addition is limited to approximately 1/2 ion per unit cell, which corresponds to 1/4 occupancy of the large cavities. Greater additions lead to the formation of other crystal phases, viz., divalent metal aluminosilicates of which celsian (BaO.Al.sub.2 O.sub.3.2SiO.sub.2) is illustrative.
In contrast, where monovalent metal ions, for example the alkali metal ions, provide the charge balance, their addition can be in such amounts as to occupy all of the large cavities in the cordierite structure. However, as was observed in U.S. Pat. No. 3,926,648, supra, the inclusion of large amounts of alkali metal ions commonly reduces the thermal stability of the products. That is, those substituted cordierites exhibit a lower liquidus temperature and reduced refractoriness when compared with classic cordierite. Moreover, the thermal expansion behavior of alkali metal-stuffed cordierites has been observed as being generally similar in character to that of stoichiometric cordierite, i.e., they demonstrate a positive a axis expansion and a c axis expansion that is quite negative, especially in the lower temperature regions.